Semiconducting composition



June 5, 1945. H H BARKER TAL 2,377,600

sEMI-coNDUcTING coMPosITIoNs Filed Feb. l0, 1943 Patented June 5, 1945SEMICONDUCTING COMPOSITION Harry H. Barker, Irwin, and Lawrence R. Hill,Wilkinsburg, Pa., assignors to Westinghouse Electric & ManufacturingCompany, East Pittsburgh, Pa., a corporation of Pennsylvania ApplicationFebruary 10, 1943, Serial No. 475,460

4 Claims. (Cl. 20L-76) This invention relates to means for preventingcorona in high voltage conductors and in electrical members disposed inair or other gaseous medium.

In order to eliminate the phenomenon known as corona, which is producedwhen air or other gaseous medium is subjected to certain criticalelectrical stresses, it has been heretofore proposed to apply to thesurfaces of electrical members semi-conducting or resistance paints toreduce the voltage gradient to below that value at Since electricalapparatus must operate satisfactorily for many years without requiringcontinual maintenance and repairing, it is a requisite forsemi-conducting coatings that they retain a predetermined resistancethroughout the normal life expectancy of the apparatus to which it isapplied. One of the many disadvantages of prior semi-conductingcompositions employed to prevent corona resided in the fact that theresistance rapidly increased with ageing, whereby in a few years oftimethe composition was ineffective for the purpose of lowering thevoltage gradient along the surfaces of the electrical conductors.

Since corona is essentially a gas phenomenon, it tends to occur atsurfaces where gases come in contact with electrically charged members.Semi-conducting compositions are, therefore,

' primarily applied to surfaces exposed to air or other gases in orderto reduce the voltage gradient at such points. However, these surfacesare exposed and subject to abrasion as will occur when electricalapparatus is in operation, or is cleaned to remove dust and the like.Consequently, it is necessary that the semi-conducting composition beresistant to abrasion and the usual wear and tear that takes place atexposed surfaces. Furthermore, the composition must resist the effectsof high temperature, particularly in conjunction with the acceleratedoxidation and other chemical effects of air and moisture at elevatedtemperatures.

The object of this invention is to provide a semi-conducting materialwhich may be readily applied with commercial quality control and whichis stable when subjected to high temperatures and has good abrasionresistance.

A further object of this invention is to provide a semi-conductingcomposition capable of retaining a substantially constant predeterminedresistance when utilized at high temperatures to which dynamo-electricmachines and other electrical apparatus are subjected.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a fuller understanding of the nature and objects of the invention,reference should be had to the single figure of the drawing. which is aplot of the abrasion resistance of several resinous coating materialsheat-treated for periods of time.

This invention is directed to a combination of metallo-organic resinsemployed as the vehicle for a semi-conducting pigmentl or iiller, whichis stable at elevated temperatures for long periods of time.

According to. this invention, it has been discovered that anthracitecoal properly selected and treated is an exceptionally desirablesemiconducting pigment or ller suitable for incorporation into organicfilm-forming materials in producing a semi-conducting coating havingstability and predetermined resistivity. For the purpose of thisinvention, the anthracite coal employed should contain less than 10%volatile matter and be free from earthy matter and other undesirableimpurities. There are relatively few sources of anthracite coal suitablefor the practice of this invention. The electrical resistivity of coalsfrom different sourcesl varies greatly. It has been found that for mostapplications, two or more grades of anthracite coal must be blended insuitable proportions in order that a predetermined electricalresistivity may be secured. Six samples of anthracite coal were selectedwhich were obtained from dierent geographic locations in Pennsylvania.These samples in a dry powdered state were tested for electricalconductivity. The per cent of volatile matter and its relation toelectrical resistivity for the several coals were as follows:

Table Percent volatile matter As recd. 20 mesh. .009 megs.

The electrical resistance of the coal was determined directly. and thisvalue is not the resistivity which will be obtained when the anthracitecoal is suitably treated and incorporated in various organic lm-i'orming materials em` ployed as a vehicle. The resistivity oi.' thefinal product, however, will vary according to some proportionality tothe resistivity indicated in the tables, but the values may be of anentirely different order.

Anthracite coal may be purchased in a finely broken state such as No. 5Buckwheat inch diameter or finer). The anthracite coal. is prepared byblending two or more available anthracite coals as determined byprepared tables and experience. It is only rarely that anthracite coalfrom one source is of exactly the correct resistivity for a givenpurpose. A semi-conducting paint may be prepared from the anthracitecoal, as purchased or pre-ground to around 100 mesh, by incorporating apredetermined quantity of the anthracite coal in a suitable film-formingvehicle, such as a metallo-organic insulating varnish. The selection ofthe vehicle will depend on the stability, toughness, hardness and otherrequirements to be met by the paint when applied.

In producing a paint-like composition from a resinous substance and theanthracite coal, it has been found that the best results are obtainedwhen the anthracite coal and the resin with a suitable solvent aremilled together, for example, in a ball mill, for a period of time offrom 2 to 24 hours. While the coal and resin may be milled concurrentlyi'or the entire time, another method of achieving this result is toinitially subject the anthracite coal suspended alone in the solvent toball milling until it nearly reaches the desired degree of colloidaliineness. The resin or other vehicle is added at this time. The ballmilling of the resin, anthracite coal and solvent iS continued for atleast one-half hour to secure an intimate and homogeneous paint. Resinswhich polymerize readily are preferably thoroughly mixed with theanthracite coal by this short milling time after the resin is added.After either method oi mixing, it may be said that the coal isincorporated in the resin.

It has been found that incorporation of the anthracite pigment into themetallo-organic nlmiorming material by milling in a ball mill, tube millor other device is necessary in order to secure desirable coatingproperties to provide for uniformity of electrical resistivity. Themilling operation reduces the anthracite coal to substantially colloidalneness. A convenient method of determining the degre of comminution ofthe anthracite coal is to place a 0.01% suspension of coal in a solventcarrier, such as toluol or acetone, and determine the relative lighttransmission through a standard photo-electric colorimeter. A 5% lighttransmission is believed to correspond to an average particle size ofabout one (1) micron. Light transmission values o! from 1/4 of 1% to 10%are regarded as indicative of sumcient milling.

Tests have shown that electrical resistivity of the paint composition isbest controlled by preparing a composition with from 15% to 90% ofanthracite coal based on the total weight of coal and resin. In thisrange of proportions, small errors or changes do not produce an unduechange in the electrical resistivity. For most purposes, highlysatisfactory coatings may be secured when the composition consists ofapproximately 50% of anthracite coal and 50% of resin. The amount orproportion of solvent is optional in order to secure proper brushing,spraying, or

dipping consistencies. Inasmuch as a volatile solvent is employed, thesolvent will evaporate on further processing the coating when applied toan electrical member.

It a particular resistivity is desired, the blend of anthracite coalshould be initially selected in order to give predetermined values inpreference to varying the resistivity by varying the quantity ofanthracite coal pigment incorporated into the resin. Not only is thestability of the semi-conducting coating improved when larger quantitiesof pigment are employes, but the variation of resistivity with voltageis not objectionable when from 15% to 90% of anthracite coal pigment ispresent in the resin. An undesirable rate of change of resistivity withapplied voltage usually does occur when any carbonaceous pigment ispresent in a iilm in amounts oi' about 10% or less.

In producing semi-conducting coatings for electrical members which areto be subjected to high temperatures, it has been discovered that theeffect of the oxygen and moisture in the atmosphere as well as thethermal effect tends to cause harmful changes when the usual resins areemployed. In some cases, the resinous material changes in its propertiesdue to continuing polymerization to a brittle stage with adverseresults. Cracking of the resin film and other undesirable physicalchanges may take place. Corona will take place at such cracked or brokenportions, thereby defeating the intended purpose of the coating.

It has been discovered that a metallo-organic vehicle or resin may beemployed in combination with anthracite coal to produce coatings capableof maintaining predetermined resistivity for prolonged periods of timeat temperatures of above 100 C. to 225 C. and higher. Metallo-organiccompounds which have been found to be particularly satisfactory for thispurpose are the phenyl ethyl silicones. The phenyl ethyl siliconesproduce a polymerized resinous material when subjected to certaincatalysts under elevated temperature conditions, the polymerizedsilicone being soluble in a solvent when in an intermediate stage o!polymerization. A solution of such intermediate polymerized silicone maybe applied to members, and when subjected to heat treatment, furtherpolymerization takes place, and the phenyl ethyl silicone gives a hardresistant lm or coating capable of withstanding high temperatures forprolonged periods of time.

Polymerized phenyl ethyl silicone is believed to consist of a series ofalternate silicon and oxygen atoms attached to one another with a phenyland ethyl radical attached to the silicon atoms. In some cases, twophenyl groups may be attached to one silicon atom, and in others twoethyl groups to one silicon atom. The average composition indicates thatapproximately one phenyl and one ethyl group is present in the polymerfor each silicon atom. The ratio o! phenyl to ethyl radicals may bevaried slightly in some cases.

A commercial source of phenyl-etlwl silicones for practicing theinvention is the resin sold as S-A by Dow-Corning Sales Company.

For the purpose of this invention, polymerized phenyl ethyl silicone ispreferred to other organic silicones. While nearly all the siliconesapparently have a high degre of resistance to thermal effects, not allthe silicones are equally re'- sistant to abrasion. Referring to theligure of the drawing, there is shown a plurality of curves of theabrasion resistance oi' several resinous sistivity for to i5 years orlonger when submaterials plotted against the hours at which the resinshave been subjected to baking. The

abrasion test was performed on a standard known type of abrasion tester,wherein a rotating wheel loaded with a given weight is operated incontact with a wire carrying the resin to be tested. 'I'he samples fromwhich the curves have been prepared each consisted of No. copper wirecovered with a layer of continuous filament of glass fibers. The resinwas applied as a varnish or enamel to the glass ilber covered wire andbaked for the times indicated. The total thickness of the wire samplesvaried from 0.065 to 0.073 inch. The upper curvewas that secured byapplying a standard thermosetting varnish consisting of entirely organicmaterials sold to the trade as Westinghouse Thermoset Varnish. Themiddle abrasion curve was that secured by employing the phenyl ethylsilicone polymer specified herein. While the relative abrasionresistance of the thermoset varnish appears to be superior to that ofthe phenyl ethyl silicone,r

it should be noted that the baking temperatures are so much greater incase of the phenyl ethyl silicone that the abrasion resistance of thelatter is under these conditions unexpected. Thermoset varnish could notbe subjected to temperatures of 225 C. for similar periods of time withany reasonable abrasion resistance being secured.

For comparative purposes, a methyl silicone polymer was applied to thesame types of glass fiber covered conductor. As will be noted, theabrasion resistance was almost negligible. The conductors failed underthe abrasion test in a few revolutions. If the methyl silicone treatedconductor were to be employed in electrical apparatus, vibration,particles of dust, and other atmospheric abrasives would soon cause thematerial to fail by wearing away. Accidental contacts with tools whilethe apparatus was being adjusted or repaired would likewise undoubtedlycause considerable damage. Consequently, a methyl silicone polymer wouldbe unsatisfactory on the ground of abrasion resistance.

It is believed that the phenyl ethyl silicones constitute a class ofsilicone resins that are more suitable for use as a semi-conductingpaint to be applied to exposed surfaces than other silicones. There issome evidence that this is due to the fact that the ethyl group in thephenyl ethyl silicone under appropriate reaction conditions changes to avinyl group which may be responsible for producing a tougher and moreabrasion resistant polymer than is obtained with other organicsilicones.

A semi-conducting composition was prepared by incorporating 260 parts byweight of anthracite coal in 400 parts of phenyl ethyl silicone solutioncomposed of 50% solids in toluene solvent. The composition was ballmilled two and one-half hours. The compound was brushed into 10 milthick tape of glass bers, the tape being wound on one-inch diameterglass tubes. The resistance of .the composition was determined aftersubjecting to various conditions. After baking 120 hours .at 200 C., theresistance was found to be 0.8 times that of the initial resistancesecured by baking 'one and one-half hours at the same temperature. Thecompound on the glass tape was flexible even after baking i6 hours at150 C. No hardness or undesirable brittleness was evident. Acceleratedageing tests indicate that this material should retain a resistivity ofthe same order as the initial re- Jected to normal operatingtemperatures of electrical apparatus..

The anthracite coal and phenyl ethyl silicone polymer dissolved in asolvent may be applied to conductors or electrical members in severalways. Fully assembled apparatus, such as generators, may be treated bybrushing or spraying or even dipping the apparatus in the composition.Upon drying, a uniform film or coating covering all the exposed surfaceswill be produced.

An alternate method of application having desirable features is to paintthe exposed end windings of the apparatus, for example, with a layer ofthe composition, and while wet, the applied semi-conducting paint iswrapped with a porous fabric tape, for example, of glass fibers orasbestos so that the paint will penetrate the tape and ooze through theinterstices thereof. Thereafter, the tape may be painted with a secondlayer of semi-conducting composition to cover the entire exteriorsurface. A highly durable surface coating is produced by following thisprocedure. In many cases, end windings which are lashed with a tape ortwine may be treated to produce the semi-conducting coating on thelashings to grade the potential between successive windings. Spacerblocks and the like may be likewise painted with the resin. In this way,any possible source of a potential gradient above the critical potentialfor causing corona will be eliminated.

In some cases the conducting pigment need not be entirely anthracitecoal, but wood chars or other finely divided conducting material may beadded. Wood chars prepared by heat treating wood at predeterminedtemperatures to a given resistivity, as disclosed in McCulloch PatentNo. 2,050,357, may be added up to anequal weight of the anthracite coaland both milled with the polymerized silicone to produce a suitablesemi-conducting paint.

It is a requirement that the semi-conducting coating be substantiallyuniformly effective over the entire end of an elctrodynamic machine. Theattainment of a reasonable degree of uniformity should not be toodifficult or costly. Commercial quality control has ben secured by thepractice of the invention described herein veasily and conveniently. Amaximum variation in resistivity between any portions of end windings ofthe order of 20% has been found commercially practicable. `It isbelieved that this degree of uniformity of resistivity has not beenpossible heretofore and is possible only by the use of the anthracitecoal composition set forth.

The incorporation of finely divided anthracite coal of predeterminedresistivity in a phenylethyl silicone film-forming material by suitablyball milling the whole gives a paintcapable of producing semi-conductingcoatings characterized not only by a, predetermined electricalresistivity of the order of 1 to 100 megohms, but also results in asemi-conducting coating having remarkably stable characteristics,particularly at elevated temperatures, thereby giving it a longoperating life. Once the electrical members of apparatus have beenprepared with anthracite coal semi-conducting paint of this invention,the apparatus have been prepared with anthracite life without requiringre-treatment with the semi-conducting paint. The protection of organicinsulation indefinitely against the destructive action of corona byreducing the potential gradient on the surface thereof will maintain theoperating efiiciency of the insulation for its full span of life.

Since certain obvious changes may be made in the above procedures anddifferent embodiments of the invention could be made without departingfrom the scape thereof, it is intended that all matter contained in theabove description or taken in connection with the accompanying drawing,shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. An electrically semi-conducting composition comprising, incombination, finely divided anthracite coal of colloidal dimensionsembodied in a polymerized phenyl ethyl silicone.

2. An electrically semi-conducting composition suitable for applicationto electrical members to provide for a predetermined resistance,coznprising finely divided anthracite coal of colloidal dimensions,polymerized phenyl ethyl silicone and a solvent for the phenyl ethylsilicone.

3. A composition for application to electrically insulated conductorsfor grading the potential on the insulation, comprising finely dividedanthracite coal oi' substantially colloidal tiness having less than 10%volatile matter and a vehicle for the finely divided anthracite coalcomprising a polymerized phenyl ethyl silicone and a solvent for thepolymerized phenyl ethyl silicone whereby when the composition isapplied to the insulation and dried, a .resinons film with adistribution of the finely divided anthracite coal therein is produced,the nlm having predetermined ohmic resistivity and characterized by ahigh degree of stability.

4. A composition for application to electrically insulated members forgrading the potential on the insulation, comprising from 15 to 90 partsby Weight ci finely divided anthracite coal having less than 19%volatile matter and from 85 to .le parts by weight or' pclymerizedphenyl ethyl silicone, and a solvent therefor, the composition whenapplied to the member and dried forming a film of resin with adistribution of the finely divided anthracite coal therein having apredetermined ohmic resistivity and characterized by a high degree ofstability.

HARRY H. BARKER. LAWRENCE R. HILL.

